In the prior art (Korean Utility Model Publication No. 88-2020) for which the present invention intends to improve, a plurality of longitudinal water tank groups and a plurality of lateral water tanks are disposed in two steps above a combustion chamber. A hot water tank is made to surround the longitudinal and lateral water tanks, and the lateral water tank supplies hot water to heat the rooms, while the hot water tank supplies hot water to the kitchen.
The above described conventional boiler has an advantages such that the use of the kitchen hot water is not affected by the temperature of the room heating hot water. However, the path for the convection current of water is limited within the longitudinal and lateral water tanks, and the conduit for the discharge of exhaustion gas is short. Further, heat radiating area is not large, and therefore, the thermal efficiency of the boiler is very low.